Hydrophobically modified cellulose polymers such as methylcellulose (MC) and hydroxypropyl methylcellulose (HPMC) have been used as binders in automotive substrate and diesel filter ceramic precursor batch compositions. These polymers give the batch the necessary plasticity and green strength in the forming and drying stages to produce high quality honeycomb ware. However, polymers such as MC and HPMC can undergo phase separation and subsequent gelation at a characteristic temperature. At the right temperature the polymer loses the water that surrounds the pendant methoxy side groups. This loss of hydration exposes the methoxy groups and enables hydrophobic associations to occur between the methoxy substituents of neighboring chains. This leads to phase separation and ultimately the build up of a long range network gel. (Sarkar, N., J. Appl. Polym. Sci, 24, 1073-1087 (1979); Methocel Cellulose Ethers Technical Handbook, Dow Chemical Co.; Li, L et al., Langmuir, 18, 7291-7298 (2002)). When the binder undergoes this thermal phase transition within a ceramic precursor batch, the batch becomes stiffer and the extrusion pressure increases significantly which can produce severe defects in the extruded honeycomb structure.
The thermal transition behavior of polymers like MC and HPMC can limit the extrusion process of numerous ceramic product lines. For example, production will have to significantly increase the extrusion feedrate of new diesel compositions such as aluminum titanate (AT) and advanced cordierite (AC) in the next 1-2 years due to increased demand for diesel products. However, the batch temperature increases with feedrate due to increased shear heating in the extruder. Ultimately, throughput reaches a limit as the batch approaches the thermal transition temperature of the binder.